A. Field of the Invention
The present invention relates to an interior coating for a color CRT (cathode ray tube).
B. Description of the Prior Art
A typical color CRT is illustrated in FIG. 1. A panel 1, the inside of which is covered with a fluorescent layer 10, is sealed to a funnel 2, the inside of which is covered with an electroconductive graphite coating, through the melting of frit glass in a furnace at about 450.degree. C. to create a sealed unit. The neck 7 of the funnel 2 contains three electron guns 3 that generate and direct streams of free electrons in three separate electron beams. A frame 5 is attached to the inside of the panel 1 so as to support a shadow mask 4, which serves as electrodes filtering electron beams by three colors. A deflection yoke 8 surrounds the neck of the CRT at the junction of the neck 7 with the funnel 2. Reference numeral 5' represents a contact spring.
With the color CRT as constructed above, when an image signal is transmitted to the electron guns 3, the cathodes of the guns 3 generate electrons that accelerate towards and are focused on the back surface of the panel 1.
The electron beams 9 are deflected by the magnetic field of the deflection yoke 8, which is installed around the neck 7, and pass through the slots of the shadow mask 4 which is suspended by the frame 5. Passing through the slots, each of the beams 9 is filtered to strike only its intended color dot. Thus the filtered electron beams 9 strike the three sets of colored phosphor dots in the fluorescent layer on the inside surface of the panel so as to produce the desired pixel colors.
An inner shield 6 is installed behind frame 5 so that the electron beams are not deflected under the influence of the terrestrial magnetic field when they pass through the slots of the shadow mask and arrive at the fluorescent layer.
Referring to a CRT 20 in FIG. 2, a panel 1 is sealed to a funnel 2 at a fusion junction. Internal and external conductive coatings, 21 and 22, are applied to the inner and outer surfaces of the funnel 2, and serve as a condenser. When high voltages are applied to a cavity 23 of the CRT 20, an image is produced on the face of the tube.
In the manufacture of the tube, the conductive coating is made from a mixture of graphite, an adhesive (water glass), and a disperser. Modern conductive coatings are treated with metal oxides to produce a surface with increased electric resistance.
A conventional conductive coating has been applied with a brush or a sponge, or may be sprayed, or applied with a deposition or flow coating method. The flow coating method in which deposit of the conductive coating is easily achieved at a time is the most widely used of the methods to coat the inside of the tunnel. When depositing the conductive coating on the outside of the funnel, a brush or a sponge is used.
Of the constituents cf the conductive coating, graphite is a conductive material that permits the current applied through the cavity to flow across the conductive coating towards the electron guns.
An adhesive consisting of potassium silicate and sodium silicate makes it easy to bond graphite and metal oxides to the glass surface of the funnel.
A disperser is added to the conductive coating to disperse the graphite and metal oxides in the glass water mixture containing distilled water.
The metal oxides added to the conductive coating with graphite, are nonconducting substances to increase the electric resistance. Usually ferric oxide (Fe.sub.2 O.sub.3) or titanium dioxide (TiO.sub.2) are used.
When using a conductive coating which contains no metal oxides, contaminants in the electron guns can cause electric sparks. Thus generated high current between 600 and 1000 .ANG. can damage the conductive coating which is in contact with the electron guns and the components of the electric circuit of the CRT.
To solve the above mentioned problem, existing conductive coatings made from the mixture of graphite, an adhesive, and a disperser are treated with nonconductive material such as metal oxides, ferric oxide or titanium dioxide.
The specific gravities of ferric oxide and titanium dioxide added to reduce overcurrent are higher than that of graphite so that layer separation occurs in a conductive coating solution left as it is or deposited on the funnel: heavier ferric oxide and titanium dioxide settle first and the lighter graphite is concentrated on the top.
When the conductive coating with the graphite layer is concentrated on the top after being deposited and dried, the resistance decreases with the increase of the conductivity, thereby providing the same problem as the conductive coating with no metal oxide. In addition, excessive time is required to disperse the settled metal oxides and the conductive coating is not uniformly deposited.
When the conductive coating is deposited by means of a brush or a sponge, the depositing process is complex and the conductive coating is not uniform. The spray painting has the limitation that the coating may be stained in the dispersed condition of the graphite slurry.
Using the deposition or the flow coating method the coating tends to be applied to undesired areas, which requires additional process to remove the undesired coating and also wastes conductive materials. The inside surface of the funnel may also have the electric resistance properties that arc not uniform because the coating streams down the inside surface of the funnel and thus the coating's thickness varies from the upper part of the funnel to the yoke section.
The difference in potential between the spring and the conductive coating can cause internal discharging when the CRT is turned on or off, especially when the electric resistance is as high as above 5 K.OMEGA. at the contact area between the contact spring and the conductive coating. Thus the difference in potential destroys the conductive coating in contact with the spring. Due to the damaged coating, high voltage applied to the cavity of the CRT cannot flow uniformly across on the inside surface of the funnel, and the panel cannot display any images.